The conventional sheathed heating element uses a coiled wire as the resistance element that is able to elongate and contract as the electric element is turned on and turned off. The coiled wire is able to expand and contract in the manner of a coiled spring because of its coils without unduly stressing the resistance element itself or its connection with an electrical terminal which is usually a welded connection. The coiled resistance element accommodates different thermal—expansion coefficients of the different metals used for the sheath and for the heating resistance element. Typically, the sheath will be made of stainless steel, copper or aluminum while the resistance element will be an alloy having nickel, chrome or the like therein. Moreover, the external sheath and the internal resistance wire operate at different temperatures with the internal resistance element operating at a higher temperature than the outer sheath which is being cooled by the medium in which it is immersed whether the medium is air, a liquid, or other material. The resistance element operating at a higher temperature typically expands more than the outer protective sheath and hence the coil accommodates this difference in expansion between the sheath and the resistance element.
The conventional manner of making such coiled resistance elements comprises winding the resistance element wire on a mandrel and removing the wound wire coil from the mandrel; welding terminals to the ends of the wire coil and bringing the coiled wire and an external sheath tube together within a loading machine at which the insulating material is loaded between the internal coiled wire and outer sheath. Typically the insulating material is a granular or powdered material such as magnesium oxide. The filled tube is then extruded with the diameter of the sheath tube being reduced substantially and the length of the tube and internal coiled wire being increased greatly. The extruding pressures compact the insulating material greatly. When the coil wire is of fine gauge, it stretches easily during the extruding process, but as the wire diameter becomes large it becomes difficult to stretch the wire coils with conventional extruding pressures.
Also, as the diameter of the wire becomes larger, it is also more stiff and cannot be easily wrapped about a small diameter mandrel. For example, using conventional coiling equipment, wire diameters of 0.0285 inch are difficult to wind and wire diameters of 0.032 inch or larger are too stiff to be wound on the small diameter arbor selected for the size of coil desired. Given this limitation in size of the round wire diameters and using conventional resistance element wires, the largest wire that was able to be wound on the mandrel size needed for this application wire had a resistance of about 0.12 ohm/inch in the extruded, finished heating device. Some applications require a resistance lower than 0.12 ohm/inch. For example, in a very long heater, e.g., 200 inches or more which is to be operated at 120 or 240 volts, the resistance of the heating element in the final heater is desired to be about 0.05 ohm/inch which is substantially below the 0.12 ohm/inch of the largest coiled wires type of heating element for this mandrel diameter of heater assembly.
Heretofore, for these applications, requiring a lower ohm/inch heater than can be produced with coiled wire for the cross-sectional diameter of the heating element, a straight, uncoiled wire of larger diameter was used. This straight wire, sheathed heater is commonly referred to as mineral insulated or MI cable. A shorter length of wire is used in the MI cable. A significant shortcoming of this MI cable is that it does not accommodate thermal expansion of the heater very well and hence tends to stress the resistance element itself and also to stress the welded terminal joints, either of which can lead to a premature failure of the heater. Long life is an expected and necessary characteristic of sheathed, electrical resistance heaters and premature failures are unacceptable from a commercial marketing of the heater.